1. Field of the Invention
This invention relates to hydraulically-operated percussive devices of the type having a piston/striker adapted to impart a succession of blows to a tool and associated with damping means which operate with a dashpot snubbing action to limit the forward working strokes of the piston/striker. Such damping means are commonly employed with devices in the form of road breakers and rock drills, often being inoperative during normal working but coming into operation to prevent overstroking of the piston/striker in the event that the tool either breaks through or is lifted off the work.
2. Description of the Prior Art
The damping means may employ a step on the piston/striker which enters a dashpot recess at one end of a chamber in which hydraulic fluid pressure acts on the piston/striker to produce return stokes thereof. In this case the pressure acts on the damping step and whereas damping, particularly with a high power tool, demands a large piston damping area the requirements of efficiency demand a low differential piston area on which the return pressure acts. The latter is particularly the case with tools of the class in which said chamber is continuously pressurised during tool operation, with the result that the return piston force is applied at all times even during the forward working strokes and hence must be kept as low as practicable.
In order to meet the conflicting requirements of a large piston damping area and a small piston return area, without a need for disadvantageous compromise between these areas, two arrangements have been proposed. U.S. Pat. No. 3,887,019 discloses a device with a flange on the piston/striker which is freely movable in said chamber and has different annular end areas, one such area providing the large area damping step and the difference between these annular areas providing the differential piston return area. While this is a satisfactory solution to the problem it necessitates a large diameter chamber with adequate clearance around said flange and complicates the design. In the other arrangement disclosed in U.S. Pat. No. 3,490,549 a completely separate damping chamber with its own end seals is provided, the piston/striker having a damping flange movable in this chamber. This increases the length of the device by a complete stroke plus the dimensions of the additional sealing arrangements, and thus also has the disadvantage of introducing structural complications as well as additional weight.